Integrated circuits, including dies, for example, imager dies such as charge-coupled-devices (CCD) and complementary metal oxide semiconductor (CMOS) dies, have commonly been used in photo-imaging applications.
Imager dies, such as the CMOS imager die, typically contain thousands of pixels in a pixel array to be used in a single chip. Pixels convert light into an electrical signal that can then be stored and recalled by an electrical device such as, for example, a processor. The electrical signals that are stored may be recalled to produce an image on, for example, a computer screen.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, all of which are assigned to Micron Technology, Inc. The disclosures of each of the forgoing patents are hereby incorporated by reference in their entirety.
FIG. 1 illustrates a block diagram of an imager die 110 having a CMOS imager device 108 formed therein. The CMOS imager device 108 has a pixel array 114 that comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixel cells of each row in pixel array 114 are all turned on at the same time by a row select line (not shown), and the pixel cells of each column are selectively output by respective column select lines (not shown). A plurality of row and column lines are provided for the entire pixel array 114. The row lines are selectively activated in sequence by a row driver 101 in response to a row address decoder 102. The column select lines are selectively activated in sequence for each row activation by a column driver 103 in response to a column address decoder 104. The CMOS imager device 108 is operated by a control circuit 105, which controls the address decoders 102, 104 for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry 101, 103 to apply driving voltage to the drive transistors of the selected row and column lines.
Output signals typically include a pixel reset signal Vrst, taken from a charge storage node after the pixel is reset, and a pixel image signal Vsig, which is taken from the charge storage node after charges generated by an image are transferred to the node. The Vrst and Vsig signals are read by a sample and hold circuit 106 and are subtracted by a differential amplifier 107, which produces a difference signal (Vrst-Vsig) for each pixel cell that represents the amount of light impinging on the pixels. This difference signal is digitized by an analog-to-digital converter 109. The digitized difference signals are then fed to an image processor 111 to form and output a digital image. In addition, as depicted in FIG. 1, the imager die 110, and, thus, CMOS imager device 108, may be included on a single semiconductor chip.
Imager dies, e.g., imager die 110, are typically packaged and inserted into imaging devices such as, for example, a digital camera. FIG. 2 illustrates a cross-sectional view of one conventional imager die package 100. The illustrated package 100 includes the imager die 110 positioned on a substrate 112. As discussed above, the imager die 110 has an imager device 108 (FIG. 1) having a pixel array 114 formed therein. In the package 100, the imager die 110 typically has a transparent element 116 over the pixel array 114. The transparent element 116 is typically attached to the imager die 110 by an adhesive material 118, or any other material that can support the transparent element 116 over the pixel array 114.
In operation, light radiation enters the transparent element 116 of the imager die package 100. The transparent element 116 filters out IR radiation that can cause color shifts in the pixel array 114. Light radiation is then adsorbed, and image signals are created by the pixel array 114, which converts the photons from light radiation to electrical signals, as discussed above with respect to FIG. 1. Wire bonds 122 conduct electrical output signals from the imager die 110 to wiring on the substrate 112, which, in turn, connects to external circuitry.
In displaying an acquired image, a display structure, for example, a computer screen, will display a complete image only if the complete image is captured by the pixel array 114. For example, if the pixel array 114 were subjected to white light from a light source 130 (FIG. 5), the expected display image would be an all white image. If the pixel array 114 is unable to capture the entire image, however, an incomplete display image 300, illustrated in FIG. 3, will be displayed on a computer screen. The illustrated display image 300 appears to have a completely white surface with a “hole” or defect 124 created by the failure to capture the complete image (in this case, a white light) from the pixel array 114. For the sake of clarity, only a portion of the full image having the defect 124 has been illustrated, and has been magnified for illustrative purposes.
The defect 124 may be a result of two separate and distinct causes. One possible reason for the defect 124 is that the pixel array contains one or more non-functional pixels (i.e., the array is defective). A top-down view of a section 400 of a pixel array having a defective pixel is illustrated in FIG. 4. The illustrated section 400 has a single non-functional pixel 126. The non-functional pixel 126 may receive light, but may not be able to convert the light into an electrical signal that can be stored and recalled as described above with respect to FIG. 1, resulting in a defect 124 (FIG. 3).
Imager die packages having non-functional pixels 126 will likely be segregated into groups by the manufacturer, depending on the number of non-functional pixels each package contains. These groups of packages, can be salvaged and used for various applications, or, if necessary, can be discarded completely. For example, the imager die package 100 (FIG. 1) having the non-functional pixel 126 could be used in applications that do not require the highest resolution, and would likely not be used in high-end applications such as, for example, professional photography equipment. Alternatively, the imager die package 100 could be discarded altogether if it contains a significant number of non-functional pixels.
A second reason for the image defect 124 (FIG. 3) may be related to particulate contamination of the transparent element 116 (FIG. 2). FIG. 5, for example, illustrates a particle 128 present on the transparent element 116. The particle 128 may have resulted from the fabrication processing of the imager die 110. During testing or operation of the imager die package 100, the particle 128 prevents light from the light source 130 from reaching the corresponding pixel (represented by the “X”) of pixel array 114, resulting in the defect 124 illustrated in FIG. 3.
Unlike the imager die having a non-functional pixel 126 (FIG. 4), the imager die 110 of FIG. 5 is fully functional. The output of electrical signals by the fully functional pixel array will, nevertheless, result in a display image similar to the display image 300 illustrated in FIG. 3. Because the particle 128 causes the fully functional array to produce a defective output image, the fully functional pixel array will be segregated into the groups of dysfunctional arrays discussed above that may be salvaged and used in low-end applications, or discarded altogether. Discarding fully functional pixel arrays results in lower yield, and increases the overall costs of production.
Accordingly, there is a need and desire for a method and apparatus for testing the quality of an imager die package, and determining the level at which the defect is present.